This invention relates to gas turbine engines and, more particularly, to an improved method of operating same for reduced noise levels.
Subsonic performance improvement of a jet engine may be obtained by incorporating a fan which is, in essence, analogous to a shrouded propeller. The fan is disposed in a duct and rotatably driven by a shaft connection with a core engine and serves to pass a large volume of air through the duct, thereby increasing overall thrust. Thrust is proportional to, among other parameters, the amount of air moved and its velocity. Though the velocity of the air exiting the fan duct is low compared to that exiting the core engine, the fan often moves many times as much air as the core engine and is, therefore, the most significant contributor to turbofan engine thrust.
Simply stated, the problem is that gas turbine engines make noise, sometimes more noise than is environmentally desirable. The noise sources are varied and complex, but two substantial contributors are blade passing noises propagating forward out of the engine inlet and the noises caused by the viscous shearing between the exhaust gases and the relatively quiescent surrounding atmosphere. Complete solution of the problem has been evasive, but much effort is being expended toward that goal. In particular, it is desirable to reduce engine noise while an aircraft is airborne and in close proximity to the ground, such as during approach and landing operations.
Ideally, a choked inlet (Mach number equals 1) would prevent engine internally generated noise such as blade passing noise, from propagating forward. The principle employed is that an acoustic wave cannot propagate upstream against a Mach one flow since the wave itself can only travel at Mach one. However, a choked inlet is inefficient and produces performance problems. Therefore, it is preferred to use high subsonic inlets which, though less effective acoustically, are more efficient aerodynamically. The lower the inlet Mach number, the less acoustic suppression in the forward direction. Thus, on aircraft approach when the engine is throttled back, two significant acoustic developments occur. First, since the exhaust velocity is decreased, the shearing noise diminishes. Secondly, however, the decreased inlet Mach number causes reduced forward noise suppression and a possible net increase in overall engine-generated noise levels.
The problem facing the engine manufacturer, therefore, is to develop a method of operating a gas turbofan engine in a reduced thrust mode wherein both inlet and exhaust-related noise are maintained at a reduced level.